How to add a thick resin "clear coat" over a material?
You can use the principled shader in 2.79.

Just use the clearcoat slider, even if something is plugged into the normal socket it still does NOT affect it. (There is another socket for clearcoat normal to add bumps (realism)). 

This is it turned off:



This is it turned on:



I cranked the normal map up way to high, I was just using it to make it more clear.